The Role of Nitric Oxide in Development and Pathogenesis of Biotrophic Phytopathogens – Downy and Powdery Mildews

Abstract

Abstract Nitric oxide (NO) and reactive nitrogen species are involved in multiple signaling and regulatory pathways across all organisms. This review provides a survey of current knowledge on the role of NO in the development and growth of plant pathogens, highlighting biotrophic parasites depending on an intimate relationship with their hosts. In plant–pathogen interactions, the major focus has been dedicated to the sources, metabolism and functions of NO as a key component of plant signaling and immunity. On the side of pathogens, much less information has been gathered about the sources and fate of NO. Recent findings suggest that the germination, oriented growth and active penetration of filamentous microorganisms, such as oomycetes and fungi, to the host tissues involve endogenous production of NO and reactive oxygen species (ROS). Methodological approaches available so far constraint more detailed studies of NO metabolism in downy and powdery mildews, as these obligate biotrophs cannot be detached from their host plants. We summarize data obtained using two model pathosystems, lettuce – downy mildew and tomato – powdery mildew, which confirm the NO production by biotrophic parasites aiming to penetrate host tissues but omit their priming. The need for a fine balancing of NO and ROS levels during their development and pathogenesis is proposed and discussed in relation to differences in biological and pathophysiological characteristics of both groups of pathogens.